In many types of circuits employing MOS transistors, the variations in threshold voltage among the transistors must be accurately controlled to insure proper operation. U.S. Pat. No. 4,180,807, issued to Charles W. Eichelberger et al. Dec. 25, 1979, for example, discloses a charge transfer circuit in which the magnitude of charge packets transferred via metal-oxide-semiconductor (MOS) transistors varies as a function of the difference between the threshold voltages of the transistors. A significant threshold voltage difference causes errors in the charge packet transfer. Such threshold voltage differences in the charge transfer circuit are minimized by arranging the threshold sensitive transistors in close proximity.
Flat panel displays using liquid crystal display (LCD) or similar devices may include thin film MOS transistors deposited on the glass substrate of the LCD. While there is threshold voltage variation in bulk type MOS transistors such as used in the aforementioned U.S. Pat. No. 4,180,807, the variation is limited by the small size of the integrated circuit structure. A LCD display is significantly larger and the threshold variations in thin film transistors over the larger surface are much greater. As larger size panel displays with more gray scale levels are developed, the variation of the characteristics of the thin film transistors over the panel tends to increase. To obtain gray scale variation in an LCD type display, an analog voltage is impressed on the LCD. Attaining larger number of gray scale values, however, is severely limited by the capability of thin film transistors in analog circuitry with respect to threshold voltage related characteristics, subthreshold slope and carrier mobility.
As is well known in the art, accurate LCD displays utilize the conversion of digital data into an analog signal that is impressed across a light varying medium. The digital to analog conversion, however, has required accurate or well matched devices. The variation of threshold related thin film transistor characteristics has made it difficult to provide useful digital to analog converters over the extensive surface of a large panel display. Dependency on such threshold voltage characteristics has been avoided through the use of high speed gray scale counters or complicated high speed clock and control signal arrangements. But such alternative circuitry is more complex, requires a higher transistor count and results in lower wafer yield. The present invention provides accurate threshold voltage compensation for a large number of transistors in a large area circuit such as an array of digital to analog converters in a liquid crystal display.